Democratic lawmakers, including Manchin, have criticized Whitaker's oversight of the Mueller probe.

"What raises my concerns is a person that's been so vocal against the investigation that was going on is [put] in charge a day after the [midterm] election," Manchin told CBS. "I think that gives concern to every senator, Democrat and Republican. We are a country — the rule of law is everything.

"Looking like it's been tilted one way or the other is wrong."

"What raises my concerns is a person that's been so vocal against the investigation... now putting in charge a day after the election... I think that gives concern to every senator, Democrat and Republican. We are a country, rule of law is everything." -- @Sen_JoeManchinpic.twitter.com/hvGE7ebcN4

Trump has repeatedly called the Russia investigation a "witch hunt," and on Wednesday said that he could fire everyone in Mueller's office if he wanted. He said he would not take that step for political reasons, however.